This invention relates to a glass substrate for use in a magnetic recording medium, such as, a hard disk, a magnetic recording medium having the glass substrate, and a method of manufacturing the magnetic recording medium and the glass substrate.
An interface between a magnetic head and a magnetic disk has become a key technology for improving recording capacity in a technical field for magnetically recording (writing) and reproducing (reading).
It is necessary to excessively reduce a flying height of the magnetic head which is float over a surface of the magnetic disk to improve recording density.
However, when the record/reproduce (write/read) operation is carried out in the known CSS (Contact Start Stop) method, the magnetic head often sticks to the magnetic disk with a low flying height of the magnetic head. Herein, it is to be noted that this phenomenon is generally called xe2x80x9chead stictionxe2x80x9d.
Suggestions conventionally have been made about a variety of texture techniques to prevent such stiction of the magnetic head. A representative suggestion has been made about a method of forming a surface of an Al/NiP plating substrate into a rough surface by mechanically polishing (mechanically texturing) the surface in Japanese Unexamined Patent Publication (JP-A) No. S62-273619. Further, another suggestion has been made about a method of depositing a thin-film having the rough surface on a glass substrate by the use of the known sputtering process in Japanese Examined Patent Publication (JP-B) No. H04-62413 or a method of forming the rough surface by the use of the chemical etching process in Japanese Examined Patent Publication (JP-B) No. H07-101507, Japanese Examined Patent Publication (JP-B) No. H07-153059 when the glass substrate is superior in flatness in comparison with an aluminum substrate.
In particular, an etching process is carried out by the use of etching liquid after performing a moisture and heat insulation process for a glass substrate in Japanese Examined Patent Publication (JP-B) No. H07-153059. Thereby, repeatability of formation of projections and uniformity of a projection height, which conventionally have caused problems in the texturing technique due to the chemical etching method, have been improved.
Meanwhile, a glide height recently has reached 1.2xcexc inch or less to improve the recording capacity.
However, the method of forming the texture, which has been conventionally suggested and described above, is the texture technique on the condition that the glide height is equal to about 8xcexc inch.
Therefore, even when the conventional texture forming method is applied for the recent magnetic disk which records (writes) and reproduces (reads) with the low flying height, it is difficult to obtain the magnetic disk which simultaneously satisfies sufficient electromagnetic conversion characteristic and stiction preventing effect of the magnetic head.
In this case, the conventional glide height was equal to about 8xcexc inch. Therefore, the surface state (the surface morphology) of the magnetic disk (the substrate) could be sufficiently evaluated by the known thally step. Herein, the surface roughness is measured by scanning a contact needle having radius of several xcexcm (for example, 2.5 xcexcm) along the surface.
However, when the flying height becomes 1.2xcexc inch or less (1 inch=25.4 mm) which is recently required, it is difficult to judge whether or not the surface state of the glass substrate can realize prevention of the stiction of the magnetic head by the use of the conventional thally step.
In the meantime, attention recently has been paid for a magnetic disk apparatus of a load/unload system (a ramp load system) in stead of the CSS system. In such a load/unload system, a magnetic head travels on a data area of the magnetic disk via an arm after the magnetic disk is rotated and is driven different from the CSS system.
Consequently, it is unnecessary to provide the texture for preventing the stiction when the magnetic head halts. Further, the surface roughness of the disk surface becomes small, and the flying height of the magnetic head for the magnetic disk also becomes small. As a result, it is possible to reproduce with high recording density.
Thus, it is required that the medium surface is flat, the projection height is low, and variation of the projection heights is small (values of Rmax/Ra and Rp/Ra are small) in the load/unload system (ramp load system) in comparison with the CSS system.
Specifically, it is necessary that Rmax falls within the range between 3 and 15 nm, Ra falls within the range between 0.2 and 2.5 nm, Rmax/Ra falls within the range between 3 and 15, or Rp falls within the range between 1 and 7 nm, Ra falls within the range between 0.2 and 2.5 nm, and Rp/Ra falls within the range between 1 and 15.
In this case, the surface preferably has projections within the above-mentioned range, and is not completely flat in the load/unload system. In particular, Ra preferably falls within the range between 0.6 and 1.3 nm.
In the meantime, the surface roughness required for the CSS system is specified by Rmax=6-18 nm, Ra=0.7-1.5 nm, and Rmax/Ra=10-20. Further, the surface roughness is specified by Rp=3-15 nm, Ra=0.7-1.5 nm, and Rp/Ra=3-15 when the surface roughness is controlled by Rp.
It is therefore an object of this invention to provide a magnetic recording medium and a glass substrate for the magnetic recording medium which is capable of realizing a glide height of 1.2xcexc inch or less and which is capable of realizing high electromagnetic characteristic.
It is another object of this invention to provide a magnetic recording medium and a glass substrate for the magnetic recording medium which is suitable for a load/unload system by controlling a projection height, projection density, variation of projection heights, and variation of projections and which has high CSS durability characteristic suitable for a CSS system.
Inventors have paid attention to specify the surface state of the glass substrate by the use of the interatomic force microscope (AFM) in order to evaluate the surface state of the glass substrate. This is because it is impossible to identify whether or not the surface state of the glass substrate is suitable, since the resolution is low in the conventional measuring method using the contact needle method.
Based upon the above-mentioned evaluation method, it has been confirmed that height and distribution (namely, variation of the height) of each projection of fine roughness, which are formed on a principle surface of the glass substrate, are important factors to achieve the above purpose.
Further, as a result of various experiments, it has been found out that the glass substrate surface as a target or a goal can not be obtained unless specific polishing condition and surface process condition are properly combined. This invention is performed based upon this analyzed result.
Specifically, inventors have discovered that the trace, along which the abrasive grain passes, tends to be formed as the island (peak) when the surface is processed by the hydrosilicofluoric acid (which may be referred to as hydrofluosilic acid or hexafluorosilicic acid) after polishing by the polishing material containing the free abrasive grain.
Although this mechanism is not clear, the load in the polishing step by the free abrasive grain is applied to the surface of the glass substrate. Consequently, the network of Sixe2x80x94O is (systematically and) structurally changed, and nonuniformity occurs in the remaining stress distribution by the structural change.
As a result, the etching rate due to the hydrosilicofluoric acid becomes slow in the portion having relatively high remaining distortion (namely, a trace portion along which the free agrasive grain passes). This is assumed to be the above-mentioned mechanism.
An application is previously filed on the basis of this analyzed result in Japanese Patent Application No. H10-233261.
It has been found out that the projection height can be reduced, and the projection density can be reduced by heating the glass substrate before processing the surface by the hydrosilicofluoric acid in addition to the above-mentioned analyzed result. Thereby, this invention has been completed.
(First Invention)
In a method of manufacturing a glass substrate for a magnetic recording medium for forming a predetermined roughness according the first invention, a principal surface of the glass substrate is precisely polished by the use of polishing material containing free abrasive grain.
Thereby, remaining stress distribution for a portion of a polishing trace due to the free abrasive grain is generated on the surface of the glass substrate.
Subsequently, a surface process is performed for at least the principal surface of the glass substrate by the use of hydrosilicofluoric acid.
Thereby, a portion having relatively high remaining distortion in the generated remaining stress distribution is decided as an island portion.
In this case, the glass substrate is heated after precisely polishing before performing the surface process by the use of the hydrosilicofluoric acid.
(Second Invention)
In a method of manufacturing a glass substrate for a magnetic recording medium for forming a predetermined roughness according the second invention, a principal surface of the glass substrate is precisely polished by the use of polishing material containing free abrasive grain.
Thereby, remaining stress distribution for a portion of a polishing trace due to the free abrasive grain is generated on the surface of the glass substrate.
Subsequently, a chemical processing (which may be referred to as chemical surface process) is performed for at least the principal surface of the glass substrate by use of hydrosilicofluoric acid.
Thereby, a portion having relatively high remaining distortion In the generated remaining stress distribution is decided as an island portion.
In this case, the glass substrate is heated by dipping the glass substrate in heated solvent after precisely polishing before chemically processing the surface.
(Third Invention)
The chemical surface process comprises either one of an etching process by the use of solution containing hydrofluoric acid, solution containing hydrosilicofluoric acid, and alkali solution in the second invention.
(Fourth Invention)
Heating temperature in the heating process step falls within the range between 30xc2x0 C. and 180xc2x0 C. in any one the first invention through the third invention.
(Fifth Invention)
The heating process is carried out by the use of at least one selected the group consisting of hot water, heated sulfuric acid, heated glycerin, and heated phosphoric acid in any one of the first invention through the fourth invention.
(Sixth Invention)
The glass substrate contains at least alkali metal oxide and alkali earth oxide, and the content of the alkali earth oxide is not exceeding 3 mol % in any one of the first invention through the fifth invention.
(Seventh Invention)
The glass constituting the glass substrate contains SiO2 between 58 and 75 weight %, Al2O3 between 5 and 23 weight %, Li2O between 3 and 10 weight %, and Na2O between 4 and 13 weight % as main components in the sixth invention.
(Eighth Invention)
The glass contains SiO2 between 62 and 75 weight %, Al2O3 between 5 and 15 weight %, Li2O between 4 and 10 weight %, Na2O between 4 and 12 weight %, and ZrO2 between 5.5 and 15 weight % as main components, and the weight ratio of Na2O/ZrO2 falls within the range between 0.5 and 2.0 while weight ratio of Al2O/ZrO2 falls within the range between 0.4 and 2.5 in the seventh invention.
(Ninth Invention)
The chemical strengthening process is carried out after the surface process due to the hydrosilicofluoric acid in any one of the first invention through the eighth invention.
(Tenth Invention)
In a method of manufacturing a magnetic recording medium, at least a magnetic layer is formed on the principal surface of the glass substrate manufactured by the method claimed in any one of the first invention through the ninth invention.
(Eleventh Invention)
In a glass substrate for a magnetic recording medium for use in a load/unload system, the glass substrate has a principal surface, and surface roughness of the principal surface is specified by Rmax=3-15 nm, Ra=0.2-2.5 nm, and Rmax/Ra=3-15.
In this event, Ra is representative of a center-line mean roughness, and Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point.
(Twelfth Invention)
In a glass substrate for a magnetic recording medium for use in a load/unload system, the glass substrate has a principal surface, and surface roughness of the principal surface is specified by Rp=1-7 nm, Ra=0.2-2.5 nm, and Rp/Ra=1-15.
In this case, Ra is representative of a center-line mean roughness, and Rp is representative of a maximum height of a highest point.
(Thirteenth Invention)
In a magnetic recording medium having at least a magnetic layer on a glass substrate for use in a load/unload system, the glass substrate has a principal surface, and surface roughness of the principal surface is specified by Rmax=3-15 nm, Ra=0.2-2.5 nm, and Rmax/Ra=3-15.
In this event, Ra is representative of a center-line mean roughness, and Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point.
(Fourteenth Invention)
In a magnetic recording medium having at least a magnetic layer on a glass substrate for use in a load/unload system, the glass substrate has a principal surface, and surface roughness of the principal surface is specified by Rp=1-7 nm, Ra=0.2-2.5 nm, and Rp/Ra=1-15.
In this event, Ra is representative of a center-line mean roughness, and Rp is representative of a maximum height of a highest point.
According to the first invention, the glass substrate is heated after the polishing process due to the free abrasive grain before performing the surface process by the use of the hydrosilicofluoric acid.
Thereby, the remaining distortion, which is generated on the surface of the glass substrate by the polishing process due to the free abrasive grain, is relieved.
Consequently, the projection height can be reduced in comparison with such a case that the heating process is not carried out. Further, the projection density, the variation of the projection heights, and the variation of the projections can be reduced.
Therefore, the magnetic recording medium, which is suitable for the load/unload system, can be stably manufactured by controlling the projection height and the projection density, and the variation of the projection heights. In such a load/unload system, the projection height and the projection density, and the variation of the projection heights fall within the predetermined range .
Further, the magnetic recording medium, which satisfies the high CSS durability characteristic and which is suitable for the CSS system, can be stably manufactured.
As mentioned above, inventors have discovered that the trace, along which the abrasive grain passes, tends to be formed as the island (peak) when the surface is processed by the hydrosilicofluoric acid after polishing by the polishing material containing the free abrasive grain.
Although this mechanism is not clear, the load in the polishing step by the free abrasive grain is applied to the surface of the glass substrate. Consequently, the network of Sixe2x80x94O is (systematically and) structurally changed, and nonuniformity occurs in the remaining stress distribution by the structural change. Herein, the nonuniformity means that the remaining distortion of the portion of the trace, along which the free abrasive grain passes, becomes large in comparison with the remaining distortion of the peripheral portion of the trace.
As a result, the etching rate due to the hydrosilicofluoric acid becomes slow in the portion having relatively high remaining distortion (namely, a trace portion along which the free agrasive grain passes). This is assumed to be the above-mentioned mechanism.
Moreover, the heating process is carried out before performing the surface process by the use of the hydrosilicofluoric acid. Thereby, the remaining distortion is relieved, and the difference of the etching rate caused by the remaining distortion becomes small.
Consequently, the projection density and the variation of the projection heights, which give an affect for flying travel of the magnetic head, are reduced in comparison with such a case (see FIG. 1) that the heating process is not carried out, and further, a fine projection having a low projection height is formed, as illustrated in FIG. 2.
In this case, when process conditions (concentration, temperature, dipping time) due to the hydrosilicofluoric acid are changed, the projection height can be controlled to a certain degree. However, the projection density can not be controlled.
Therefore, it is preferable to perform the heating process before the process due to the hydrosilicofluoric acid like this invention compared to this method because the projection height and density can stably and accurately controlled for the low flying travel of the magnetic head.
Namely, the remaining distortion due to the free abrasive grain formed on the surface of the glass substrate is relieved by the heating process before the process due to the hydrosilicofluoric acid.
Consequently, the projection is not formed in the region having a relatively small remaining distortion due to the polishing step.
On the other hand, the remaining distortion becomes small, and the height of the formed projection becomes small in the region having a relatively high remaining distortion. The projection density can be changed by controlling the condition of the heating process.
The method of the heating process in the first invention is not particularly restricted. There are exemplified a method in which the glass substrate is dipped in the heated solvent, a method in which the glass substrate is subjected in atmosphere (air, vacuum) heated by an oven and a method in which a light ray (for example, wavelength (infrared rays and ultraviolet rays) which absorbs for the glass substrate) is irradiated for the glass substrate as the heating process.
Among them, the method, in which the glass substrate is dipped in the heated solvent, and particles for the glass substrate can be removed at the same time with the heating process, is superior from the viewpoint of quality and stability.
This is because when the particles exist on the surface of the glass substrate during the process due to the hydrosilicofluoric acid, only the portion of the particles is left without the etching to form the projection, and the surface roughness is not reduced on the whole.
Further, the remaining polishing material causes to form the projection. Therefore, it is desirable that the remaining polishing material can be simultanousely removed by the solvent. Such solvent includes sulfuric acid and organic acid (phosphoric acid, formic acid, acetic acid, propionic acid, acrylic acid, oxalic acid, glycolic acid, glyceric acid, lactic acid, gluconic acid, succinic acid, adipic acid, and the like).
Further, the hydrosilicofluoric acid used during processing the surface of the glass substrate of this invention has weak etching force (slow etching rate) as compared to hydrofluoric acid solution which contains hydrofluoric acid or potassium fluoride and which is conventionally used as the etching liquid.
Consequently, it is possible to precisely control the surface roughness. Silicofluoric acid (H2SiF6) is typically used as the hydrosilicofluoric acid.
The hydrosilicofluoric acid process may contain the other acid (hydrofluoric acid, sulfuric acid, hydrochloric acid, nitric acid) and commercially available washing materiel (natural washing material, surfactant, alkali washing material) with fine quantity in order to enhance the etching (washing) effect.
Further, the process condition of the hydrosilicofluoric acid is mainly determined in dependency upon concentration of the hydrosilicofluoric acid, immersing time into the hydrosilicofluoric acid, temperature of the hydrosilicofluoric acid.
The hydrosilicofluoric acid is formed by dissolving the silicofluoric acid into water. The concentration of the hydrosilicofluoric acid indicates the concentration in which the silicofluoric acid is dissolved in the water.
The concentration and the temperature of the hydrosilicofluoric acid relate with the etching rate (the specific range will be explained later) while the immersing time into the hydrosilicofluoric acid relates with the obtained roughness and the process time of the step.
The process condition of the above-mentioned hydrosilicofluoric acid is suitably adjusted based upon the roughness of the formed surface roughness. However, it is preferable from controllability of the surface roughness that the immersing time into the hydrosilicofluoric acid falls within the range between 50 and 600 sec and the temperature of the hydrosilicofluoric acid falls within the range between 15xc2x0 C. and 60xc2x0 C.
The concentration of the hydrosilicofluoric acid preferably falls within the range between 0.15 and 3.0 weight %.
When the concentration of the hydrosilicofluoric acid is not exceeding 0.15 weight %, the etching effect or the washing effect for the glass substrate is lowered. Consequently, the desired surface roughness can not be obtained.
When the concentration exceeds 3.0 weight %, it is difficult to control the surface roughness with high accuracy because the etching rate became quick. Consequently, the glass substrate for the magnetic recording medium having stable quality can not be obtained. This is not preferable.
Inventors have found out that the surface roughness of the glass substrate before the surface process gives large effect for the height distribution (variation) of the islands (peaks) on the substrate surface which is finally obtained to stably manufacture the glass substrate for the magnetic disk of this invention which is required to be controlled the surface roughness with high accuracy.
Inventors have enthusiastically researched this case. As a result, it is preferable that the surface of the glass substrate before the surface process is in the mirror state. Specifically, it is found out that Ra falls within the range between 0.1 and 1.0 nm, more preferably, that Ra falls within the range between 0.1 and 1.0 nm, and Rmax falls within the range between 1 and 20 nm.
According to the second invention, the glass substrate is heated by dipping the glass substrate in heated solvent after precisely polishing before chemically processing the surface.
Thereby, the magnetic recording medium, which can realize the glide height of the 1.2xcexc inch or less and realize the high electro-magnetic conversion characteristic, can be stably manufactured from the same reason as the first invention.
In this case, the etching material used for the chemical surface process is not particularly restricted. There are exemplified a method (dipping, spraying and the like) which utilizes etching liquid, such as, hydrofluoric acid, hydrosilicofluoric acid, hydrofluoric acid-fluoride mixed solution, hydrofluoric acid-inorganic acid mixed solution, and hydrofluoric acid-organic mixed solution, and a method in which the etching process is performed by contacting vapor of the hydrofluoric acid with the surface of the glass substrate.
As mentioned before, the chemical surface process or the surface process due to the hydrosilicofluoric acid is carried out such that the portion having relatively high remaining distortion in the remaining stress distribution generated for the portion of the trace due to the free abrasive grain in the polishing process of the glass substrate is decided as the island portion.
This invention positively utilizes such phenomenon, and thereby, the a predetermined surface roughness can be obtained. Further, the predetermined surface roughness can be realized by performing the heating process. Although this mechanism has been described before, the other mechanism, which may occur this phenomenon, is naturally within the scope of this invention.
According to the third invention, the glass substrate is dipped in the heated solvent of the solution containing hydrofluoric acid, the solution containing hydrosilicofluoric acid, and the alkali solution in the second invention. This method can remove the particles for the glass substrate at the same time with the heating process, and is superior in the quality and stability from the same reason as the above.
In this case, cerium oxide (CeO2), alumina (Al2O3), colloidal silica (SiO2), iron oxide (Fe2O3), chromium oxide (Cr2O3), zirconium oxide (ZrO2), titanium oxide (TiO2) are exemplified as the free abrasive grain use in this invention.
A particle diameter (size) of the free abrasive grain can be suitably adjusted in dependency upon a desired surface roughness. An average particle diameter preferably falls within the range between 0.02 xcexcm and 3.0 xcexcm.
Preferred density of the island portions and a tip shape of the island portion, which contacts with the magnetic recording medium, can be obtained by selecting such a range of the particle diameter. Consequently, higher CSS durability can be obtained in the substrate for the magnetic recording medium.
When the particle diameter is not exceeding 0.02 xcexcm, aggregation of the polishing material readily occurs, and the number of the remaining substances after the washing step becomes high. This is not preferable. On the other hand, when the particle diameter exceeds 3.0 xcexcm, the roughness after the etching process becomes excessively large. This is not also desirable.
According to the fourth invention, the heating temperature in the heating process step preferably falls within the range between 30xc2x0 C. and 180xc2x0 C.
When the heating temperature is not exceeding 30xc2x0 C., the heating process time becomes long, and production tact is extended. This is not preferable. On the other hand, when the heating temperature exceeds 180xc2x0 C., the solvent, which endures the heating process of long time, is limited, and a large equipment is necessary to perform the process. This is not also desirable.
Further preferred range of the heating temperature falls within the range between 60xc2x0 C. and 120xc2x0 C. For example, the heating process time may be suitably adjusted in accordance with the kinds of used solvent in the case of the solvent. Specifically, the heating process time may fall within the range between 30 and 600 sec.
According to the fifth invention, the heating process is preferably carried out by the use of at least one selected from the group consisting of hot water, heated sulfuric acid, heated glycerin, and heated phosphoric acid. Among them, the heated sulfuric acid is desirable because the variation of the surface roughness becomes small. Namely, contaminants attached to the glass substrate can simultaneously removed during the process due to the heated sulfuric acid.
The concentration falls within the range between 5 wt % and 99 wt %, the heating temperature falls within the range between 30xc2x0 C. and 180xc2x0 C., and the process time falls within the range between 30 sec and 600 sec as the conditions processed by the heated sulfuric acid.
In this event, the concentration of the used sulfuric acid is preferably higher, and dense sulfuric acid having 75 volume % or more, and more preferably, 95 volume % or more, is desirable.
Further, the contaminants attached to the glass substrate can be also removed by the heated phosphoric acid.
The heating temperature falls within the range between 30xc2x0 C. and 90xc2x0 C., the process time falls within the range between 60 sec and 600 sec, and the concentration falls within the range between 0.1% and 50% as the conditions processed by the heated phosphoric acid.
Alternatively, organic acid other than the phosphoric acid may be used. The phosphoric acid is superior in operability in comparison with the sulfuric acid. Further, the sulfuric acid is superior in an effect for reducing the roughness in comparison with the phosphoric acid.
According to the sixth invention, the glass substrate preferably contains at least alkali metal oxide and alkali earth oxide, and the content of the alkali earth oxide is not exceeding 3 mol %.
Namely, it is assumed that an exchange reaction occurs between H+ contained in the water and alkali ion (Na+, Li+) contained in the glass in the polishing step due to the free abrasive grain of the glass substrate surface.
By this exchange reaction, a hydration layer, which is readily etched, is formed. In the hydration layer, OH was attached to Si or Al which forms a network of the glass by the exchange reaction. It is assumed that the stress distribution was formed for the hydration layer in accordance with the stress distribution applied by the free abrasive grain, and the roughness was formed in dependency upon the etching rate.
Herein, it is to be noted that the etching rate is small at a portion having large stress while the etching rate is large at a portion having a small stress.
From such a mechanism, at least the alkali metal oxide is necessary to form the hydration layer in the glass substrate. Further, it is required that the content of the alkali earth oxide, which prevents the exchange reaction of the alkali ions for forming the hydration layer, is not exceeding 3 mol % (not exceeding 2.4 weight %), as disclosed in Japanese Patent Application No. H11-233209.
The glass substrate of the sixth invention preferably contains SiO2 between 58 and 75 weight %, Al2O3 between 5 and 23 weight %, Li2O between 3 and 10 weight %, and Na2O between 4 and 13 weight % as main components like the seventh invention.
Further, it is desirable that the glass does not contain alkali earth metal oxide, such as, CaO or MgO to remarkably form the island (peak) by the mechanism mentioned above.
In particular, it is preferable in the eighth invention that the glass substrate is an aluminosilicate glass which contains 62-75 weight % of SiO2, 5-15 weight % of Al2O3, 4-10 weight % of Li2O, 4-12 weight % of Na2O, and 5.5-15 weight % of ZrO2 as the main components, the weight ratio of Na2O/ZrO2 falls within the range between 0.5 and 2.0, and the weight ratio of Al2O3/ZrO2 falls within the range between 0.4 and 2.5.
The transverse bending strength is increased, the compressive stress layer becomes deep, the Knoop hardness is excellent, and the controllability of the etching in the surface process due to the hydrosilicofluoric acid is excessively superior by chemically chemically strengthening the aluminosilicate glass.
Therefore, such an aluminosilicate glass is desirable. Herein, it is to be noted that N5 manufactured by HOYA CORPORATION is representative of the above-mentioned aluminosilicate glass.
Further, the surface process due to the above hydrosilicofluoric acid is performed twice. Moreover, the different hydrosilicofluoric acid concentrations are used in the respective steps. Thereby, the fine surface roughness on the substrate surface can be controlled.
It is preferable that the chemical strengthening process is carried out after chemical surface process or the surface process due to the hydrosilicofluoric acid (the ninth invention). Herein, the known chemical strengthening methods are used as the above chemical strengthening method without limitation.
For example, the low-temperature ion exchange method, in which the ion exchange is performed in the region which does not exceed the transition point temperature from the viewpoint of the glass transition point, is preferable. A fused salt used for the chemical strengthening includes potassium nitrate, sodium nitrate, nitrate mixed with them.
When the above surface process due to the hydrosilicofluoric acid is performed immediately after the glass substrate surface is chemically strengthened, the remaining distortion formed by the free abrasive grains on the glass substrate surface is buried in the stress of the chemical strengthening by chemically strengthening. This is undesirable because the surface roughness can not be controlled.
However, the same result as the above-mentioned case can be obtained by interposing the polishing processing step due to the free abrasive grains between (immediately after the surface process due to the hydrosilicofluoric acid) the chemical strengthening process step and the surface process due to the hydrosilicofluoric acid as the chemical strengthening stepxe2x86x92the polishing step due to the free abrasive grainsxe2x86x92the surface process due to the hydrosilicofluoric acid.
According to the tenth invention, at least the magnetic layer is formed on the principal surface of the glass substrate manufactured by the method of manufacturing the glass substrate for the magnetic recording medium, such as, the above-mentioned magnetic disk.
Thereby, the magnetic recording medium, such as, the magnetic disk satisfies the high electromagnetic conversion characteristic and the high CSS durability characteristic.
Rmax, Ra, Rp and Rq are measured by the use of the interatomic force microscope (AMF), and are defined by JIS standard (JIS B 0601).
In this case, Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point. Ra is representative of a center-line mean roughness (an average of an absolute value of deviation between a center line and a measuring line).
Rp is representative of a maximum height of a highest point (a distance between an average line and a highest point). Rq is representative of a root mean square roughness (a root of an average square value of deviation between a center line and a measuring line).
Herein, this roughness can be determined by suitably setting a measuring region. Meanwhile, it is to be noted that a roughness data of the following examples corresponds to a data of a region of 5 xcexcmxe2x96xa1.
When the roughness exceeds an upper value of Rmax and Rp, the head flying height becomes high. This is not preferable from the viewpoint of the high recording/reproducing density.
When the roughness exceeds an upper value of Rmax/Ra and Rp/Ra, the head crush or the thermal asperity readily takes place. This is not desirable. When the roughness is not exceeding a lower value of Rmax, Rp, Rmax/Ra, and Rp/Ra, the head stiction may occur and, it is impossible to manufacture the glass substrate. This is not preferable.